A semiconductor laser has many attractive applications as a light source in optelectronic equipments including high-density optical disc systems, high speed laser printers, high bit-rate optical communication systems and optical control systems. Considerable effort has been applied to improve characteristics of a semiconductor laser, especially to reduce a threshold lasing current and to increase a external differential quantum efficiency. One of promising means for improving the above mentioned characteristics is a MQW active layer to be used in the semiconductor laser. The MQW active layer commonly used in a semiconductor laser at present has such a structure that a composition, a width, barrier heights and a period of placement of each quantum well are equal respectively in the MQW active layer. However, disadvantages of the semiconductor laser containing the above mentioned MQW active layer is that carriers cannot be uniformly distributed in each quantum well, and satisfactory improvements of a threshold lasing current, an external differential quantum efficiency and other important characteristics cannot be achieved. Then, further improvements of the MQW active layer are extremely desirable.